The term “waterjet” denotes high-speed water jets generated at high static pressures with special pumps and nozzles. Such waterjets perform a wide range of useful work such as cleaning tanks, ship hulls and various structures and also cutting alloys and composite materials with computer-controlled nozzle movement. Static pressures of water as high as 80,000 pounds per square inch (psi) are generated with special motor-driven or engine-driven piston pumps and special fluid-powered pressure intensifiers, and with nozzles equipped with gem orifices. The term “waterjet technology” describes the various processes and applications of waterjets. The term “abrasive waterjet” describes a particular waterjet technology in which selected industrial abrasive particulates are added into the jet stream with special nozzles to further enhance the capability of waterjets. Very hard and difficult materials are cut or removed with such abrasive waterjets. In fact, it is the only method that can now be used to cut carbon-fiber laminates that are widely used in modern aircrafts.
The pumps and pressure intensifiers known for generating waterjets are positive-displacement piston pumps which have multiple pistons and check valves to build up the potential energy of a fluid. The energy transfer from the piston to the fluid is usually not smooth, due to factors such as fluid compressibility, the finite number of pistons in the pump, and the phase limitations. As a result, there are pressure pulsations in the output fluid. For example, a triplex crankshaft pump has only three cylinders and pistons operating at about 600 rotations per minute (rpm) and a double-acting hydraulic pressure intensifier has only two cylinders and pistons operating at about one stroke per second. These pumps are used to push or build water pressures from atmospheric to 55,000 psi or higher. The output pressure of water at the outlet of each cylinder is not phased properly with the output pressure of other cylinders to cover the entire cycle and to provide smooth pressure output. The rough power output is similar to automobile engines where the power output of a 3-cylinder engine is rougher or not as smooth as the power output of an 8-cylinder engine. Thus, if a waterjet nozzle is placed at the outlet of a triplex pump or a double-acting intensifier, the waterjet will not form a smooth stream. Instead, the waterjet will form a pulsed jet with a stream of water slugs. The water slugs are phased according to the piston motion of the pump. For example, a triplex pump operating at 600 rpm would generate a pulsed waterjet of 3×600=1800 pulses per minute. A double-acting intensifier operating at one stroke per second would produce a pulsed waterjet of 60 pulses per minute.
However, in waterjet applications, nozzles are not positioned next to the pump. Tubes or hoses are used to transport the pressurized water from the pump to a remote or distant nozzle. Inside the tubes or hoses the pressure pulsations in the water is damped and only a portion remain at the nozzle. In many applications, the residue pressure pulsations present no problem but in double-acting intensifiers there may be a problem. Due to the very low stroke rate and the extreme pressures involved, water at the nozzle of an intensifier pump system may have pressure pulsations too high for applications such as abrasive waterjet cutting of composites. An additional pressure attenuator may be required to further damp out the pressure variations. In such applications, the smoothness of cut surface may be related to or a function of the pressure pulsation of the waterjet.
In many waterjet applications, a pulsed waterjet can be more effective than a continuous waterjet when each is at an identical pump power level. One reason is the mitigation of waterjet interference when a waterjet impacts a flat surface. When a continuous waterjet impacts a hard surface, the waterjet rebounds from the surface and collides with the incident waterjet. As a result, a significant portion of the waterjet energy is wasted. In a pulsed waterjet, the water slugs impact the surface individually and the energy of each slug of water has time to dissipate. If the waterjet slugs are phased properly, waterjet interference can be completely avoided. With a pulsed waterjet, the impact pressure on a surface can be greater if the mass of each water slug is greater. Reducing waterjet interference is one reason why waterjetting is widely applied today in industrial cleaning processes, such as by spinning a nozzle assembly at a high speed. Many waterjets generated at known pump pressures are supersonic, and it is difficult to avoid waterjet interference. Rotating a nozzle assembly at a high speed requires a rotating joint with good seals. The durability of such high-pressure seals is a maintenance issue in industrial processes. An impacting power of a waterjet is also reduced when the nozzle is rotating at a high speed.
There are many known investigations using pulsed waterjets for a wide range of jobs. One benefit of a pulsed waterjet is to remove materials, such as concrete, that have significant granular structures of materials. The waterjet pulses can better penetrate into pores of the porous structures, to rupture the structure and wash away the debris. Similar benefits of pulsed waterjet have been reported with coating removal. There are other benefits of using pulsed waterjets.
Even with the benefits of pulsed waterjets, the method is not applied widely today because the pulsed waterjet processes reported in several publications have not been commercialized. One highly publicized known pulsejet technology is not now commercialized, presumably because components involved in that particular pulsejet technology are not matured or there were technical difficulties not overcome. It is difficult to design an on-off valve for use with high-pressure water as the working fluid. To produce a pulsed waterjet at a nozzle is extremely difficult due to many factors. It is difficult to interrupt the flow of water at very high pressures.
Only some known pulsed waterjet processes are applied commercially, including one that uses an ultrasonic transducer placed at the tip of a waterjet nozzle to generate forced pulses at 20,000 cycles per second. Electrical energy is introduced into the nozzle assembly to generate the axial vibrations and forced waterjet pulses. Up to 1 kilowatt of electrical energy may be required to overcome the static water pressure at the nozzle. With this pulsed waterjet process it is possible to remove coatings at static pressures considerably lower than those associated with a conventional continuous waterjet. This 20 kHz pulsed waterjet process is not widely applied because of shortcomings and also the required electricity to power its nozzle. Mixing electricity and water in a handheld piece of field equipment is not a safe practice.
Pulsed waterjets are normally generated with available pumps. Once the pressure pulsations are dampened with tubes and hoses it can be difficult to recreate pressure pulsations at a waterjet nozzle. It is also difficult to interrupt the water flow at very high pressures. Problems, such as water hammer effect and metal fatigue, can occur if the flow interruption is not handled properly.
A process that allows a pulsed waterjet to be generated at a nozzle at a wide range of water pressures is valuable to the entire waterjet technology and would have applications in shipyards and concrete structure repairs and in everyday cleaning applications. It is particularly valuable if the process requires no energy from external or outside sources and requires no use of a heavy component with uncertain durability. This invention can be used to provide a waterjet process that produces a genuine pulsed waterjet by tapping a very small amount of water energy to produce waterjet pulses at a controllable frequency and at a wide range of static pressures. The apparatus and process of this invention will be valuable to waterjet technology and its use in industry.